The overall objective of these experiments is to determine the biochemical mechanisms involved in the switch from larval to adult hemoglobin synthesis, which occurs in erythrid cells during metamorphosis of Rana catesbeiana tadpoles. Three major experimental aaproaches to the problem have been taken. (1) An examination of RNA extracted from erythroblasts following the in vivo administration of tri-iodothyronine (T3) to tadpoles, in an attempt to identify the earliest RNA products synthesized at the time of the hemoglobin switch. (2) Characterization of histone proteins during maturation of tadpole and frog erythroid cells and during the induction of frog hemoglobin synthesis in erythroid cells by treatment of tadpoles with T3 in vivo, since histones may be implicated in the control of gene transcription. (3) An examination of ribosome structure in tadpole and frog erythroid cells, in an attempt to determine if erythroblast maturation or T3 treatment of tadpoles results in changes in the ribosomal structural proteins or ribosomal RNAs of erythroid cells. During the current grant year, we have concentrated on the first two of the objectives listed above. In addition, we have begun studies on oxygen utilization and transport in phenylhydrazine-induced anemic tadpoles, with the aim of determining how an organism without red blood cells transports oxygen to its tissues.